Type 'SelectMenuItem' according to 'as'

[smockle]

Defines SelectMenuItemProps as a discriminated union.

Follow-up testing: Does this mean the as prop is required? If so, can/should we default to either a or button?

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[emplums]

Awesome thanks @smockle! Nope, as is not required. If it's not set, the tag used in the styled components function is used (an anchor tag) :)